The solar energy industry is expanding at a rapid pace. Much of that expansion is due to increases in residential and small commercial photovoltaic (PV).installations. Increasingly these installations are directly connected to the utility grid without the use of batteries. Inverters are the power electronics equipment that convert DC electricity produced by PV panels (collectively a PV array) into the AC required by a utility grid.
Islanding is a phenomena where the grid becomes de-energized due to a power outage or other event, and a distributed generation device (a solar inverter, for example) continues to produce power, resulting in an unsafe condition for utility line workers. Underwriters Laboratories (UL) requires a distributed generation device to disconnect from the grid within two seconds after the grid voltage goes down. Sometimes a distributed generation device will “hold up” the grid voltage as it continues to push current through a low impedance load that happens to be balanced with the output of the inverter, essentially masking the grid fault at its terminals and the inverter will continue to produce power. This rare but possible situation is called islanding when the voltage at an inverter's terminals does not go below safety set points (and thus the inverter does not shut down), even though the grid is no longer present.